In immersion microlithography, as in other types of microlithography, an image of a desired pattern is transferred by a beam of exposure light to a suitable substrate. In many types of microlithography systems the pattern is defined by a reticle or mask. The reticle is illuminated by an illumination beam to form a patterned imaging beam. The imaging beam passes through a projection-optical system that shapes and conditions the beam as required to form the pattern image on a suitable substrate such as a semiconductor wafer, glass plate, or the like. For exposure, the substrate normally is held on a movable platform called a “substrate stage,” and the reticle normally is held on a movable platform called a “reticle stage.” The stages undergo controlled motion relative to each other as exposure of the substrate progresses. So as to be imprintable with the pattern image, the substrate usually is coated with a light-sensitive material called a “resist.”
To perform accurate exposures, the microlithography system is equipped with detectors and sensors that ensure, for example, proper alignment of the reticle and substrate with each other and with the system optics. Respective detectors and sensors are placed at various locations throughout the microlithography system, including on or in proximity to the reticle stage and the substrate stage.
To obtain better imaging resolution microlithographic exposures are normally performed at shorter wavelengths of exposure light. The incessant demands of forming increasingly more and smaller active-circuit elements in micro-circuits has generated a relentless demand for microlithography systems that can make exposures using shorter wavelengths. The currently most advanced microlithography systems that are commercially available perform exposures using deep-ultraviolet light produced by excimer lasers. The wavelength range for this light is approximately 150-250 nm, which is generally in the “deep-ultraviolet” or “DUV” range, wherein a favorite current wavelength is 193 nm. Very few materials are transmissive to this and other DUV wavelengths. Since optical glass is not transmissive to DUV, the projection-optical system and other system optics are usually made of fused silica (amorphous quartz).
As the world awaits a practical “next-generation” lithography system capable of making exposures at wavelengths substantially less than the excimer-laser-produced DUV wavelengths, substantial effort is being directed to urging more imaging performance from systems employing excimer laser light. In this effort, surprisingly good results have been obtained from excimer-laser-based systems that utilize “immersion” projection optics. These “immersion-microlithography” systems exploit a principle used in light microscopy, in which improved image resolution is obtained by interposing a liquid (having a refractive index substantially greater than of air) between the specimen and the objective lens. In immersion-microlithography systems an immersion liquid is interposed between the end of the projection lens and the substrate surface on which the projection lens forms the image. Whereas light-microscopy apparatus can readily accommodate use of an immersion liquid (usually an oil) in this manner, accommodating an immersion liquid in a microlithography system is more problematic, especially without actually degrading imaging performance or causing other problems.
Currently, in most immersion microlithography systems, water is generally used as the immersion liquid. As an immersion liquid, water has many desirable properties. It has a refractive index (n) of approximately 1.44 (compared to n=1 for air), and it is transmissive to the wavelength of exposure light currently being used (λ=193 nm) in immersion microlithography. Water also exhibits high surface tension, low viscosity, good thermal conductivity, and no toxicity, and the optical properties of water are well known.
To supply immersion liquid in an immersion-microlithography system, the projection-optical system is provided with a nozzle assembly (some configurations are aptly called “showerheads”) situated at or near the end of the projection-optical system adjacent the substrate. The nozzle assembly is configured to discharge the immersion liquid and to recover excess immersion liquid as required to maintain a desired amount of the liquid at the desired location in the space between the projection optics and the substrate surface.
Substrate stages used in immersion-microlithography stages typically have several optical sensors used for alignment and image evaluation. In each such sensor, a respective “optical window” normally separates the actual sensor element from the environment of the substrate stage, and detection light passes through the optical window to the sensor element. Many of these sensors are located so as to be situated at or close to the edge of the substrate carried on the stage. Thus, with a substrate stage of an immersion-microlithography system, there are instances in which the upstream-facing surface (incidence surface) of an optical window for a sensor may be contacted, at least intermittently during exposure of a substrate or exchange of substrates, by the immersion liquid.
Use of water as an immersion fluid has revealed that contact of the immersion liquid with the incidence surface can have any of several undesirable consequences. For example, such contact can cause formation of bubbles in the body of water contacting the incidence surface, especially whenever the body of water and the window are experiencing relative motion. Also, such contact can result in formation of droplets of immersion water that remain behind on the incidence surface after the body of immersion water has passed over the window. In either event, bubbles or droplets usually interfere with the function of the sensor located downstream of the window. Additionally, droplets can also perturb the fluid body remaining in the nozzle assembly if the stage motion subsequently brings the droplets and fluid body together again, thereby producing further disruption of the fluid body. These problems with bubble and droplet formation tend to be more pronounced at higher stage velocities, which is unfortunate because higher material throughput from the microlithography system usually requires higher stage velocities.
To reduce adverse consequences of contact with immersion water, the incidence surface of a conventional optical window for a sensor associated with the substrate stage is usually flush (coplanar) with the upper surface of the substrate resting on the substrate stage. Also, the incidence surface is coated with a film of a “hydrophobic” (“water-hating”) substance. A substance that is hydrophobic resists being wetted by water. Hence, droplets of water placed on the film tend to assume a “beaded” configuration rather than spreading out onto the film surface. The presence of a hydrophobic surface reduces the likelihood that the fluid body will be perturbed by motion over the surface, thereby leading to droplet formation. More generally, a water droplet placed on a hydrophobic film exhibits a “contact angle” (θ) with the film surface of greater than 90°. Conventionally used hydrophobic-film materials are polytetrafluoroethylene (PTFE; a type of TEFLON®) and certain silane compounds such as fluoroalkylsilanes. These materials are normally applied to the incidence surfaces as very thin films to ensure that the films do not excessively block transmission of light through the window.
Although water currently is widely used as an immersion liquid in immersion microlithography, certain aspects of water are not entirely favorable. One aspect is its index of refraction. Desirably, the immersion fluid has an index of refraction that is equal to or greater than the index of refraction (approximately 1.6) of the objective element in the projection-optical system actually contacted by the fluid. Water has n=1.44, which has allowed its use as an immersion fluid for 193-nm immersion lithography at the 45-nm half-pitch node, but n=1.44 is inadequate for use in immersion lithography aimed at producing finer features (38-nm half-pitch node and below). Second, water tends to absorb into and partially dissolve resist applied to a substrate surface. As a water droplet on a resist surface evaporates, the bit of resist that had been dissolved in the droplet is left behind on the resist surface, usually at a different location than originally. This redeposited resist can significantly alter the topology of the resist surface and can cause problems with measurements (such as autofocus measurements) performed at the substrate surface. Third, water readily evaporates, which increases the concentration of water vapor in the vicinity of the substrate stage and of the various interferometers used for determining stage position. This change in vapor in the paths of the interferometer laser beams can introduce errors in measurements performed by the interferometers. Also, water vapor can damage delicate optical surfaces such as the reflective surfaces of mirrors used in the various interferometers.
Also, with conventional optical windows used in immersion microlithography systems in which water is used as the immersion fluid, the thin film of conventional hydrophobic substance applied to the incidence surface is easily damaged by high-intensity DUV exposure light and is not physically durable. Consequently, the thin film tends to have a short lifetime under actual-use conditions. Surfaces that are more durably “phobic” to the immersion liquid are needed.
In view of the limitations of water, new immersion liquids for immersion microlithography are currently being sought. The search is difficult because very few substances have the requisite high transparency to DUV light and a sufficiently high index of refraction to DUV light. High transparency is advantageous for several reasons, including maximizing the exposure light incident on the photoresist, minimizing photo-degradation of the fluid, and minimizing temperature increases of the fluid. Recent work has revealed promising results with certain saturated hydrocarbons, especially certain cyclic alkanes. For example, cyclohexane has an index of refraction of 1.55 at 193 nm, compared to approximately 1.44 for water. French et al., “Second Generation Fluids for 193 nm Immersion Lithography,” Proceedings SPIE 6154:15, 2006. But, the new immersion liquids pose certain tradeoffs with respect to water. For example, compared to water the new liquids tend to be more viscous, have lower surface tension, and be more difficult to contain between the objective element and the substrate. They also tend to “wet” many surfaces more readily than water, resulting in a higher likelihood of droplets and films being left behind after passage of the liquid over a surface (such as of an optical window, which can interfere with sensor performance). Thus, the liquids pose new challenges with respect to making the incidence surfaces of optical windows sufficiently “phobic” to the liquids.